The present invention concerns a steel for use in high strength pinion shaft for manufacturing a pinion shaft used in automobile steering systems, as well as a manufacturing method thereof.
In existent steering systems, rack & pinion systems utilizing an oil pressure have been predominant in which pinion shafts are used for the systems. The pinion shaft serves as a part for transmitting a rotational torque when a driver operates a steering wheel and converting rotational motion to linear motion by meshing with a geared portion of a rack bar, and it is one of important parts in the steering system.
The pinion shaft is manufactured by the combination of steel species and surface hardening heat treatment, for example, using case hardened steel (for example, JIS SMnC420, SCM420) and applying carburization hardening and tempering, or using carbon steel (for example, JIS S45C) or tough and hard steel (for example, JIS SCM440, SCM445) and applying high frequency hardening and tempering.
However, carburization hardening and tempering to the case hardened steel involves a problem of increasing the heat treatment cost and occurrence of heat treatment strains or abnormal heat treated layers.
On the other hand, in a case of applying high frequency hardening and tempering to carbon steels or tough and hard steels, while the cost is reduced and strains are decreased compared with carburization hardening and tempering to the case hardened steel, it results in a problem of increasing the cost by refining in a case of using a material applied with a refining treatment in order to ensure the inner hardness of the pinion shaft and easiness for high frequency hardening. On the other hand, in a case of using a not-refined steel instead of the material applied with the refining treatment, it results in a problem of lowering the performance of the pinion shaft because of low impact strength.
As the not-refined steel for use in shaft such as pinion shaft, JP-A No. 09-195000 discloses a not-refined steel containing: C: 0.20-0.50%, Si: 0.05-0.70%, Mn: exceeding 0.60 and up to 1.00%, S: 0.01-0.07%, V: 0.02-0.50%, N: 0.002-0.03%, P: 0-0.050%, Cu: 0-0.30%, Ni: 0-0.30%, Cr: 0-1.00%, Mo: 0-0.30, Al: 0-0.050%, Pb: 0-0.30%, Ca: 0-0.0100%, Te: 0-0.10%, Bi: 0-0.100% and the balance of Fe and inevitable impurities in which fn1≧0 and fn2≦0; fn1=C+(Si/10)+(Mn/6)+5N+1.65V+(Cr/3)−0.6, fn2=[C/(fn1+0.6)]−0.6.
Further, gear cutting is conducted in the manufacturing steps of the pinion shaft. Depending on the material used upon gear cutting, the geared surface is sometimes roughened to deteriorate the tooth form accuracy due to the effects of hardness and tissue (particularly, in those not applied with pre-heating treatment such as refining and annealing). When the form accuracy is lowered, since the state of the meshing surface in the tooth portion of rack and pinion is worsened, the wear resistance or pitting resistance is sometimes deteriorated. Further, depending on the lowering for the tooth form accuracy, state of friction at the gearing surfaces changes greatly to result in a problem of lowering the feeling in steering.
Further, for coping with the energy saving demand in view of recent global ecological problems, electromotive type power steering utilizing motors (EPS) have been developed and tended to be used more and more instead of hydraulic power steerings.
EPS sometimes have assist mechanisms different from those in the existent steering systems and, particularly, in a type of assisting the rotational torque of the pinion shaft, since larger force exerts on the meshing portion for the teeth portion of the rack and pinion, compared with the existent systems, the working conditions tend to become severer compared with the existent systems.
In a case where steels free of the refining treatment, that is, not-refined steels are put to gear cutting as they are by a hob or the like, this results in a problem that the gear cut surface becomes roughened to deteriorate the tooth fore accuracy and lower the wear resistance or pitting resistance. Further, in a case where high frequency hardening is applied to usual not-refined steels, since they have high ferrite content, ferrite remains in the high frequency hardened layer under usual high frequency heating conditions, failing to obtain a predetermined surface hardness thereby resulting in a problem of lowering the wear resistance and the pitting resistance. In a case where heating is conducted at higher temperature or for longer time as high frequency heating conditions in order not to remain ferrite, this results in a problem that the depth of the hardened layer increases to cause large heat treatment strains or crystal grains growth to lower the strength. Further, in a case of applying high frequency hardening to existent not-refined steels, it results in a problem of failing to obtain desired strength to torsion, bending, impact torsion or impact bending.